Recently, development of polymer materials which decompose in the natural environment has been desired in view of the worldwide problems of environmental protection. Especially, plastics which are decomposed by microorganisms are greatly expected in the industries as environmentally adaptable materials or functional materials of new type.
It has been well known that aliphatic polyesters have biodegradability and representative are poly-3-hydroxybutyrates (PHB) which are produced by microorganisms and polycaprolactones (PCL) which are synthetic polymers. Biopolyesters mainly composed of PHB have excellent environmental adaptability, but are poor in productivity and have a limit to be substituted for general-purpose plastics. As for PCL, those of high polymerization degree capable of being molded into films have been obtained, but they have a melting point lower than 65.degree. C. and are inferior in heat resistance and cannot be applied to wide uses.
Furthermore, it is known, for example, in "Polym. Sci. Technol.", Vol.3, Page 61 (1973) that aliphatic polyesters obtained from aliphatic dibasic acids and aliphatic dihydroxy compounds have biodegradability. However, these polymers are poor in heat stability and in addition, accompany decomposition reaction at the time of polycondensation and hence, only those of about 10000 in number-average molecular weight can be obtained.
JP 4-189822A, U.S. Pat. No. 5,310,782 and EP (A1) 0572,256 report that aliphatic polyesters are treated with diisocyanates such as hexamethylene diisocyanate to increase the molecular weight thereof. These polyesters are polyester urethanes containing urethane bond formed by diisocyanates and crosslinkage in the molecules and they are insufficient in biodegradability and have a problem in appearance when molded into films.
It is known that as polyester carbonates obtained using aliphatic compounds, those which are obtained using alicyclic compounds or aromatic compounds and aliphatic compounds can be used in the form of molded articles since normally they have a high melting point or a high glass transition point. However, they are very poor in degradability with microorganisms or do not show biodegradability.
Only the aliphatic polyester carbonates obtained by ring opening polymerization using cyclic monomers are classified in biodegradable polymers. These comprise a hydroxycarboxylic acid unit and an aliphatic carbonate unit as constitutive elements and have compatibility with living body and are usable in medical field, but since they have hydrolyzability, there is a limit in using them as films, sheets or molded articles.
On the other hand, it is known to produce polyester carbonates from aliphatic dibasic acids, aliphatic dihydroxy compounds and diaryl carbonates. For example, JP 60-13811A discloses a process for producing aliphatic polyester carbonates from low-molecular weight polyester diols or polyether diols and diphenyl carbonate. However, the resulting aliphatic polyester carbonates are rubbery and soluble in tetrahydrofuran at room temperature and it is merely shown that they can be used as additives to plastics.
The aliphatic polyester carbonates composed of aliphatic dibasic acids and aliphatic dihydroxy compounds are generally low in melting point, have rubber-like properties and inferior in heat resistance and solvent resistance. Such aliphatic polyester carbonates are mainly used for starting materials for urethane as liquid low-molecular weight compounds and at present, they are utilized, for example, as adhesives, sealing agents, coating agents and additives to other resins and have never been practically used as molded articles such as films, sheets and fibers. Furthermore, it has not been known to use them as biodegradable polymers required to have practical general-purpose properties.